hemster
Global Moderator 
Particle Manufacturer
...still listening... still watching
Posts: 51,951
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Post by hemster on Dec 15, 2017 19:36:50 GMT -5
routlaw , negative feedback is a good thing! Negative feedback (aka degenerative feedback) uses a small part of the output (current or voltage) and feeds it back into the input, albeit out of phase (hence "negative") to the input signal. This has the effect of reducing some gain. However the benefits are many - including noise and distortion reduction, increasing system bandwidth and improving input & output impedances. Negative feedback also improves stability. So this is why tube amps (without negative feedback) have greater distortion and that manifests itself as the characteristic "warm, tube" sound. Edit: Wow, that was a long reach into the back of my brain to dig up some stuff stored there from electronics lectures many years ago! 
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Post by leonski on Dec 15, 2017 20:31:22 GMT -5
ALL semiconductor power devices have a measureable called RDon. This is resistance of the divice when on. And this burns power while creaating some heat. I don't see how you can avoid this. Class 'D or not.
\I've heard that 'D' amps have a THEORETICAL efficiency of 100% (output stage only) which can't be realized in practice. Efficiency of a single stage is not very meaningful, since you must actually listen to the amp.
This means, IMO, that efficiency from Plug to Speaker is what counts.
NFB is another of several '3rd rail' topics in high fidelity. Has Bruno P. found the secret to the secret sauce? Sure would be neat.
Until than? Designers like Pass apply feedback only to a single stage of an amplifier at a time and not in a 'global' fashion which is from output to input. Pass also apparently minimizes the Number of 'stages' of amplification.
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Post by sahmen on Dec 15, 2017 22:54:45 GMT -5
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Post by roadwar on Dec 15, 2017 23:36:32 GMT -5
Me too. Looking forward to January 15th. I will be forced to turn in my Secret Monoblock Society card. They came to my house demanding I surrender my "Secret Monoblock Society card" after selling my pair of XPR-1's but they discovered I still own a pair of Carver M-1.0's amplifiers (for now)... Hmmmm, your post reminded me that I still own two old Carver M 400t "cube" amps.  Looks like I'm golden as to Secret Monoblock Society membership even if I don't keep my XPA-1Ls. |
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Post by leonski on Dec 16, 2017 0:00:09 GMT -5
They came to my house demanding I surrender my "Secret Monoblock Society card" after selling my pair of XPR-1's but they discovered I still own a pair of Carver M-1.0's amplifiers (for now)... Hmmmm, your post reminded me that I still own two old Carver M 400t "cube" amps. Looks like I'm golden as to Secret Monoblock Society membership even if I don't keep my XPA-1Ls. M400t is a stereo amp. While It can easily be 'bridged' to a 500 watt mono, that is not really the best use of the amp. Output? 200x2 @8 and 250x2 @4, which tells everything I need to know about the PS and capability. I had one of these guys for 20+ years and it rocked my Magnepan MG-1, being a very good match to the power handling limit of the speaker, driving to 'fuse pop' at the very UPPER limits. Being nearly a resistive load, the panel also, apparently, had no wacky low impedance excursions which I don't think the amp would have been too happy confronting. The M400t would also, in its last installation, cause the house lights to flicker in-time to the music as it sucked the house line voltage down. Quite the 'Chinese Puzzle' to disassemble, too. The ONLY way to get a DR series mono bloc, is to order ONE module per enclosure. That has the real benefit of coming with what certainly is an overspec'd power supply which will loaf along no matter WHAT you can do to it. |
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Post by roadwar on Dec 16, 2017 9:00:59 GMT -5
I haven't used my Carvers in years. I did use them in mono mode at 500 wpc for years. Regarding the DR-1 versus DR-2; while I would like to have "monoblock" DR-1s, I just don't have the room in my relatively small HT space for two full size Emo boxes in neither the equipment rack nor at the speakers. I am confident that the single DR-2 will provide the dynamic reserve I want. I'm also thrilled (of course) that my membership in the SMS is not threatened since I still own a pair of XPA-1L monoblocks. If questioned I will deny any knowledge of this subject.  |
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Post by mgbpuff on Dec 16, 2017 11:21:12 GMT -5
Most tube amplifiers run without feedback. This is possible in part because of the lower forward gain of tube circuits. Some tube circuits developed later in the progression of tube amp design, called ultra linear, do have feedback obtained from a special winding on the output transformer. If there is no feedback then overall linearity depends on the characteristic curve of the amplifying device. Tubes in general are more linear throughout their frequency range than transistors. Therefore transistors are inherently more in need of feedback to gain the necessary linearity and because output transformers are not needed in transistor amp design, feedback from output to input is easier. It is possible to vary gain with frequency and thus correct for non linearity in an open loop fashion, which is what Nelson Pass likes to do. However such circuitry is intolerant of changes with age or temperature or loading. Feedback automatically adjusts for these types of external influences. Feedback is fairly easy to understand in linear circuts, but when it comes to non linear circuits such as class D, feedback becomes very complicated. It is in this class D area that Putzey has inventively applied feedback. Until his PWM feedback implementation design came along, class D was open loop. By the way negative feedback is an oxymoron. Feedback must be negative or it is destructive. Positive feedback causes run away instability.
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Post by mgbpuff on Dec 16, 2017 12:30:26 GMT -5
A Word About REGULATED Power SuppliesWe've mentioned that the XPA Gen3 and our new XPA-DR amplifiers use a heavy-duty Switch Mode Power Supply (SMPS).
And we've also mentioned in passing that it's a REGULATED power supply.
However, it occurred to me that a lot of you may not understand exactly what that means - and what a big deal it really is.
Amplifier Power Supplies 101
You probably know that the purpose of a power supply is to convert the 120 VAC (or 230 VAC) coming out of the wall into the voltages needed to power your audio equipment.
Virtually all of the circuitry in audio equipment runs on DC - at a variety of different voltages and current requirements.
Because they don't use much power, but are really fussy about having clean power, devices like preamps usually have regulated power supplies.
What this means is that, after passing through the transformer, and the rectifiers that convert the AC into DC, and the capacitors that filter it, the power passes through an additional regulator stage.
A regulator is an active device that acts sort of like an amplifier - except that, instead of music, its output signal is clean DC power.
The regulator removes virtually all of the last little bit of noise and ripple, makes sure that the voltage is correct, and makes sure that the voltage doesn't change, and the noise doesn't increase, when the load changes.
The drawback to old-style linear regulators is that they're only about as efficient as amplifiers.
That's fine for low-powered devices like preamps - which use so little power that wasting a significant percentage of it is no big deal.
But, when it comes to power amps, running the output stage from a regulated power supply just hasn't been practical - even though it might improve the sound quality.
Adding a regulated power supply big enough to run the output stage of an amplifier would make the whole amplifier bigger, heavier, less efficient, more complicated, and a whole lot more expensive.
And, because using a regulated power supply on a power amplifier would be so difficult, and the gains in sound quality wouldn't be immediately obvious, almost nobody does it.
At least until now.
The 3 kW Switch Mode Power Supply that powers our new XPA Gen3 and XPA DR amplifiers is fully regulated.
It delivers the same sort of super-clean tightly-controlled power normally used by the precision circuitry in a good preamp....
And enables us to run ALL of the circuitry in our power amp modules from the good stuff - including the heavy duty output stage in each.
The result is a small but significant improvement in sound quality....
(And, in case you were wondering, our modern SMPS power supply is able to deliver all of those benefits WITHOUT the drawbacks of older designs.)
So, the next time you compare one of our new amps to one of our "competitors".....
Check to see if they're using a REGULATED power supply.
I have some questions about the SMPS in the "DR" amps. Granted, the main SMPS delivers a regulated DC power supply. I assume that would be for the higher voltage rail. How is the lower voltage rail produced? By a second SMPS, or by a normal DD/DC regulator. If the later, isn't the inefficiency worse than for a linear PS with a dedicated secondary winding on the xformer? Also if a SMPS delivers power directly from the power line, how can it react intantaneously without the same capacitance storage that a linear PS requires. After all, if the impulse power required occurs during a AC line null point, there must be about the same amount of capacitive storage in either SMPA or Linear PS, shouldn't there? Where are the caps in the DR design? I don't see them in the pics! |
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Post by gsand on Dec 16, 2017 13:27:17 GMT -5
The XPA amps use a Class H topology.... which means that their output stage is Class A/B, but is powered by dual power supply rails. At very low power levels, their efficiency is very similar to Class A/B (a tiny bit better). And, at full power, their efficiency is also quite similar. However, at medium power levels, which is where most amplifiers operate most of the time, they are significantly more efficient. We can't really give you a number because it depends on the type of music you're playing, the impedance of your speakers, and how loudly you are playing them. Note, however, that they are still less efficient than a Class D amplifier..... Internally, solid state devices are "voltage operated devices" while transistors are "current operated devices". However, that really doesn't have much to do with how efficient the overall unit is. All Class A/B devices operate in a linear fashion, which means that, at certain output voltage levels, the output devices must "drop voltage while passing current" - which means that they're burning power. Class D devices are designed to switch current - and so are either "on" or "off" and so avoid "dropping power" or "burning power". This makes them more efficient. However, because a music waveform varies continuously rather than switching on and off, amplifying it is more easily suited to a Class A/B amplifier. For this reason, it can be more a more complicated and difficult task to "adapt" a Class D "switching" circuit to accurately amplify a continuously varying music waveform. This is why it's taken so long for Class D designs that actually sound good to be developed and marketed. Hmm, its always been my understanding that Class D amps are more current driven, while Class A or A/B are more voltage driven including tube amps. Regardless one thing to consider here that has not been mentioned is the fact how much more efficient these new amps are compared to heat loss generated A/B or A. I have to assume that alone might require less demands on an AC circuit. Nice to learn about the topology of the DR's but have a question about how they compare musically to the XPR-1's or the XPA-1's? Maybe Keith could add some listening evaluation/insight from his perspective. I am replacing a pair of XPR-1's with a DR 2 to free up some space. |
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Post by gsand on Jan 1, 2018 10:46:27 GMT -5
Well the countdown to the official shipping date date for the XPA DR's is 15 days away. Does anyone else have one on pre-order? I am sorta anxious and hopeful they will not disappoint and make me miss my XPR-1's...
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Post by jcz06 on Jan 1, 2018 12:54:40 GMT -5
Well the countdown to the official shipping date date for the XPA DR's is 15 days away. Does anyone else have one on pre-order? I am sorta anxious and hopeful they will not disappoint and make me miss my XPR-1's... I have a DR-3 coming once available to possibly replace 3 XPR-1s in my HT system and don't believe I will be disappointed....if not then I am going to be parting with the three big monoblocks.....the fact is they all have the 3k power supply so in reality has anyone ever needed that much juice? |
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Post by leonski on Jan 1, 2018 14:15:28 GMT -5
Only persons with VERY low sensitivity speakers, in large spaces, who like it LOUD.
Not to forget, the PS is derated because of amp output efficiency, which is seldom above 55% to 60%.
Also? Your 15 amp service is ONLY 1440 watts continuous, this being a 20% derate of the 'expected' 1800 watts.
I doubt you'll ever use 3kw of PS.
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Post by jcz06 on Jan 1, 2018 14:17:05 GMT -5
Only persons with VERY low sensitivity speakers, in large spaces, who like it LOUD. Not to forget, the PS is derated because of amp output efficiency, which is seldom above 55% to 60%. Also? Your 15 amp service is ONLY 1440 watts continuous, this being a 20% derate of the 'expected' 1800 watts. I doubt you'll ever use 3kw of PS. Every Amp I have has access to its own 20A circuit  .... |
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Post by novisnick on Jan 1, 2018 14:20:47 GMT -5
Only persons with VERY low sensitivity speakers, in large spaces, who like it LOUD. Not to forget, the PS is derated because of amp output efficiency, which is seldom above 55% to 60%. Also? Your 15 amp service is ONLY 1440 watts continuous, this being a 20% derate of the 'expected' 1800 watts. I doubt you'll ever use 3kw of PS. I know of no one that even comes close to using the power of the dark side their big amps. But IMHO it has definite benefits to headroom and sonic bliss! YMMV |
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Post by Casey Leedom on Jan 1, 2018 16:19:08 GMT -5
...I have a DR-3 coming once available to possibly replace 3 XPR-1s in my HT system and don't believe I will be disappointed.... And I for one am very interested in seeing your review on that seeing as we have the same L/C/R speakers ... :-) Casey |
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Post by roadwar on Jan 1, 2018 17:15:03 GMT -5
Well the countdown to the official shipping date date for the XPA DR's is 15 days away. Does anyone else have one on pre-order? I am sorta anxious and hopeful they will not disappoint and make me miss my XPR-1's... I have a DR-2 ordered. Looking forward to seeing what it can do. |
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Post by leonski on Jan 1, 2018 17:48:41 GMT -5
Only persons with VERY low sensitivity speakers, in large spaces, who like it LOUD. Not to forget, the PS is derated because of amp output efficiency, which is seldom above 55% to 60%. Also? Your 15 amp service is ONLY 1440 watts continuous, this being a 20% derate of the 'expected' 1800 watts. I doubt you'll ever use 3kw of PS. I know of no one that even comes close to using the power of the dark side their big amps. But IMHO it has definite benefits to headroom and sonic bliss! YMMV Yes, Burst power thru a 20 amp circuit can be pretty impressive. Continuous w/20% derate? about 2kw. And yes, again, a dedicated 20 amp circuit for EACH amp? Nice. And even though I feel it could be helpful, given the generally sad state of 'stock' cordage, I'm sure a 12ga power cord could be helpful for those BIG bursts of power. |
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Post by novisnick on Jan 1, 2018 17:58:24 GMT -5
I know of no one that even comes close to using the power of the dark side their big amps. But IMHO it has definite benefits to headroom and sonic bliss! YMMV Yes, Burst power thru a 20 amp circuit can be pretty impressive. Continuous w/20% derate? about 2kw. And yes, again, a dedicated 20 amp circuit for EACH amp? Nice. And even though I feel it could be helpful, given the generally sad state of 'stock' cordage, I'm sure a 12ga power cord could be helpful for those BIG bursts of power. Even a 15 amp circuit will bring benefits to those big bursts. |
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Looks like I'm golden as to Secret Monoblock Society membership even if I don't keep my XPA-1Ls.
